There is a trend in multi-carrier mobile communication consumer equipment towards the provision of multimode wireless services using various standards, which are continuously being updated. As the demand for personalized applications suited to diverse needs continues to grow, there is an increasing need for multimode terminals that can provide seamless connectivity between different multi-carrier modes and that can be upgraded according to user needs.
A spread-spectrum (SS) technique is often used to distribute wireless transmit signals over a wider bandwidth than the minimum required transmission bandwidth. In military applications, SS transmission may be used to avoid jamming and also to reduce the probability of detection or interception. In civilian applications, some forms of SS, such as code-division multiple access (CDMA) may be used to allow multiple users to share the same channel or spectrum. Common techniques being used are direct-sequence spread spectrum (DSSS) and frequency-hopping spread spectrum (FHSS). These common SS techniques may suffer from susceptibility to narrow and partial band interference. Multi-Carrier Spread Spectrum (MCSS) is a particular form of SS that is designed to be resistant to narrow and/or partial band interference. In one conventional form, Orthogonal Frequency Division Multiplexing (OFDM) techniques have been used for creating this spreading.
The use of wireless channels to generate secret-keys has been proposed before. However, a number of researchers have noted problems in the more general context of information encryption and noted that some on-the-fly strategies have to be adopted for achieving a higher degree of security. As a result, many proposals have reported various strategies for key agreement between communicating parties over publicly available spectra. One of the earliest works is a so-called “puzzle solver system” devised as an intelligent strategy (puzzle system) where the two communicating parties pass messages that will enable them to jointly solve a puzzle whose solution is a common key of length N. They solve the puzzle with a complexity that is on the order of N. An adversary who may have access to the communicated messages can also solve the puzzle, albeit with a complexity of order N2 or higher. Assuming that N is a large number, the latter complexity may be prohibitively large and thus significantly reduces the chance of the adversary finding the key.
Later studies proposed the possibility of using some random information that may be available to a pair of communicating parties for generating a secret-key. However, some of these proposals discovered some fundamental bounds on the rate of generation of keys when a pair of communicating parties has access to the correlated random variables due to some external source. The most natural “external source” is some characteristics of the channel response, in the time domain or frequency domain due to the reciprocity property of the propagation channel between the communicating parties. The reciprocity property states that when device A and device B are communicating, at a given time, the channel response from device A to device B is the same as the channel response from device B to device A.
Using this reciprocity property, some proposals suggested a simple and practical method of deploying the channel reciprocity in selecting secret-keys. It has been proposed that one might transmit a set of tones from device A to device B and later (but within an interval that one may assume the reciprocity holds) the same set of tones is transmitted from device B to device A. The phase difference between the tones are measured at both device A and device B and based on the measured results a key is selected. An eavesdropper at another location with device C will see different channel responses (from device A to device C and device B to device C) than the one that connects device A and device B. However, due to the channel noise, differences in measurements are likely. To resolve this problem, multiple measurements should be performed and some exchange of information between A and B is needed to minimize the probability of any mismatch of the generated keys at device A and device B.
Using channel responses and channel reciprocity, a variety of measurement techniques and characteristics of the propagation channel have been proposed to devise mechanisms for key generation. However, these proposals assume that a long key of length N is generated and the transmit data is encoded using this key. For example, the encoding is performed by an operating Boolean Exclusive OR (XOR) function on the blocks of size N data bits and the secret-key. Hence, any mismatch between the secret-keys at A and B will result in errors in detected data at both sides.
There is a need for apparatuses and methods that determine channel responses and use that channel response in a new way to develop secret keys that are self-generated at two different devices and provide fault-tolerant encryption of communication between the two devices.